For me (I don't use it), Gmail notifier is, as it's said a notifier, which is able also to launch the browser with the gmail url, but it's not a mail client, and I think the Gmail installer doesn't set the registry for mailto protocol.

I had have the same problem under Linux and wine, using Linux Thunderbird and K-Meleon under wine.

The trick I have found has been to modify the macros, and now it works, but a necessary thing to be able to do it is to know how the parameter is passed between Google notifier and K-Meleon ...

Here is the modifications in mail.kmm.
I must first explain that the initial purpose was to unbreak all the mail functions under Linux (and wine), so I made a test to recognize on which system K-Meleon is running, and a second test send the good command.

Mail_SendPageURL{
macroinfo=_("Send a link to the current page by mail");
$_windows=readfile("Z:\\etc\\group");
$_windows==""?0:$__mailto=getpref(STRING,"kmeleon.plugins.macros.mail.mailClient");
$_windows==""?&JS_hndlTop:$_Location="mailto:?subject=".$TITLE."&body=".$URL;
$_windows==""?0:exec($__mailto." ".$_Location);
}

a second example, and you'll finish the rest:

Mail_SendPageURL{
macroinfo=_("Send a link to the current page by mail");
$_Location="mailto:?subject=".$TITLE."&body=".$URL;
exec(getpref(STRING,"kmeleon.plugins.macros.mail.mailClient")." ".$_Location);
}